1. Field of the Invention
This invention relates to a processing system suitable for accurately processing a free-form surface at a high speed, and more particularly, to a numerical control apparatus for controlling a main spindle revolution on instructions of a processing program and a CAM system for adding tool contact positional information to a processing program.
2. Description of Related Art
In a numerical control apparatus suitable for accurately processing a free-form surface at a high speed, the moving path of a tool is instructed by a processing program, and the moving speed of the tool along the path is also instructed by marking an F code in the processing program. However, if the tool is moved at a speed instructed by the F code, the system may fail to maintain the processing accuracy due to error factors such as a delay of a servo system.
As a measure of preventing such disadvantages, some of recent NC apparatuses for high-speed, high-accuracy processing of free-form surfaces are configured to estimate the form of the instructed path, find the acceptable maximum speed for maintaining the processing accuracy within an acceptable error range depending upon the estimated form, and automatically limit the speed within the acceptable maximum speed regardless of the speed instructed by the F code.
Although the revolution of the main spindle is instructed by an S code in the program, there is an optimum combination between the revolution of the main spindle and the feeding speed of the main spindle depending on the cutting conditions. However, in case a system has the above-mentioned function of automatically controlling the feeding speed in accordance with the form, if these values are instructed by the F code and the S code of the program, the speed value is limited, and even though the revolution of the main spindle is constant, the speed may become far from the value instructed by the F code.
In cutting, it is generally considered to be a desirable cutting condition that the feeding amount of each blade is constant and the cutting speed, i.e. the relative tangential speed of the contact points between the tool and the workpiece, is contact.
Under that condition, the real cutting speed will be stabilized and the lifetime of the tool will be elongated, wear of the tool will be alleviated, and a stable feeding speed of each blade will be obtained. As a result, the processed surface will be improved in quality, and reduction of the processing time by improvement of the feeding speed can be expected.
Under the circumstances, the Inventors proposed a numerical control apparatus disclosed in Japanese Patent Application No. hei 7-175277 (Japanese Patent Laid-Open Publication No. hei 9-29584), which can maintain the optimum condition for combination of revolution and feeding speed of the main spindle determined by the cutting condition when cutting a free-form surface, alleviates the tool wear, and ensures high-speed, high-accuracy processing.
That is, this numerical control apparatus controls a predetermined cutting speed in peripheral velocity, depending on the contact diameter of the tool, which changes in accordance with the curved surface from time to time, to synchronize the rotation and the feeding speed. More specifically, revolution of the main spindle is changed by a main spindle revolution control means in response to a feeding speed obtained by a feeding speed determining means based on a moving form of a tool instructed by a processing program; the main spindle revolution based on a change of the contact position between the tool and the workpiece in accordance with the moving direction of the main spindle is additionally changed by the main spindle revolution control means, and the main spindle revolution is further changed by the main spindle revolution control means based on a feeding speed containing information about the main spindle moving direction obtained by the feeding speed determining means in accordance with the moving form of the tool.
However, since this function computes the contact position between the tool and the workpiece based on the moving direction of the tool instructed by the processing program, it cannot cope with changes of the contact position in particular cases.
For example, FIG. 4 shows that a ball end mill 1 is working an inclined plane 2 parallel to the moving direction of the tool while moving vertically to the drawing sheet plane. In this case, there is a deviation between the contact position designated by the program instruction, which is shown by the white point, and the actual contact position, which is shown by the black point.
FIG. 5 shows that a difference is produced in contact position depending on tools. The white circle shows that in case of a ball end mill 3 whereas the full circle shows that in case of a bull nose 4.
To overcome this problem, the processing program may instruct such a main spindle revolution that CAM maintains the optimum condition of combination of the main spindle revolution and feeding amount that are determined by the cutting condition. However, since the optimum condition of combination of the main spindle revolution and the feeding speed changes variously depending on stiffness of the tool and the workpiece, number of blades of the tool, and so on, in case CAM instructs the main spindle revolution by way of the processing program, it is annoying that the processing program itself has to be re-output every time when the optimum condition changes.